Other sections reveal the basic function of geared hubs and showed several variations of the technical implementation of different speeds. The gear structure shows the layout of a specific hub, i.e. how many gear trains it contains, how these gear trains are joined together, etc. Thus there are some important parameters, which will be described here briefly:
– Number of planetary gear trains: in the simplest case a geared hub only contains one simple gear train achieving two or three speeds. By a triple stepped planetary gear train seven speeds can be realized. More complex hubs contain several gear trains thus achieving even more speeds – but different grading as well.
– Type of gear trains: As already mentioned simple gear trains and stepped gear trains are employed, as well as combinations of both. In the Shimano Nexus Inter 4 for example four speeds are realized by a stepped planetary gear train. The Sturmey-Archer model FM achieves the same number of speeds by using two simple gear trains joined together differential.

Duomatic simple gear train
Nexus Inter 7 two-stepped gear train
Sachs Elan differential gear train

Direction of power transfer: a gear train can be operated unidirectional, i.e. only one direction of translation is applied, e.g. speed reducing OR speed increasing only. This is applied in Fichtel & Sachs Automatic or Duomatic hubs for example. A gear train can also be operated bidirectional, i.e. both directions of translation are applied, e.g. speed reducing AND speed increasing. This is mostly to be found in common 3-speed hubs. The well-known model H311 from Fichtel & Sachs achieves three speeds by using a simple gear train operated bidirectional. Model 29 achieves the same number of speeds by using a stepped planetary gear train being operated unidirectional with speed reducing ratio only.

Coupling of planetary gear trains: in hubs with several gear trains the individual gear trains are often piled behind one another, i.e. switched in series. The Shimano Nexus Inter 7 for example contains two stepped gear trains. The first one provides 2 speed reducing ratios (output 1: Planet carrier 1) and the second one provides 2 speed increasing ratios (Output 2: Ring gear 2 and input 2: Planet carrier 2). In both gear trains the planet carriers are joined together thus achieving a total ratio by multiplying the single ratios. In other cases there are some hubs containing gear trains which are differential coupled: a very close ratio for example is realized by connecting the sun gear 1 of gear train 1 with the planet carrier 2 of planet train 2 in the 3-speed model SturmeyArcher AC. As already mentioned in Planetary gears this kind of coupling leads to many diverse possibilities of gear grading.

Doppeltorpedo ring gear
Doppeltorpedo planet carrier
Doppeltorpedo planetary gear train


Mister Johnson from the USA received a patent for a two speed with one planetary gear train hub in 1895. In 1902 the first 3-speed hub was produced carrying the name ‘The three speed gear syndicate company’, which later on became Sturmey-Archer. So hubs with one simple planetary gear train were the first historic internally geared hubs and they are still produced in their millions.

One simple planetary gear train – unidirectional

The simplest geared hubs only contain one simple planetary gear train which is only operated in one direction: this only provides one direct gear and a speed reducing or a speed increasing ratio. Two speed hubs were the first geared hubs of the world, in 1898 already the first 2-speed hub was produced in England named ‘The Hub’. A few years later a hub from Wanderer appeared which can probably be found only in a museum. The ‘Doppeltorpedo’ from Fichtel & Sachs can be definitely found from time to time und is one of the most aesthetic geared hubs. The sprocket is situated directly on the ring gear and by the plant carrier as – the only – output this hub has a speed reducing ratio only. The sun gear can also be pushed into the internal toothing of the planet carrier for achieving a direct gear. In the 1960’s Fichtel & Sachs put the legendary Duomatic and Automatic hubs on the market, which only have speed increasing ratios. The sprocket is situated on the planet carrier here, which provides two outputs combined with the ring gear. For the direct gear the pawls of the ring gear are disengaged when pressing them radially inwards out of the toothing of the hub shell and the planet carrier conducts the force flow 1:1 to the shell. For achieving the fast gear the pawls of the ring gear get activated by a control ring and ‘overrun’ the pawls of the planet carrier, ‘klacking’ while rotating unloaded.

Duomatic planet carrier
Duomatic ring gear
Duomatic control ring

The Sachs Orbit (combination hub) achieved between 10 and 14 gears with a gear rim of 5 to 7 sprockets by the built-in 2 speed gear. In this case, the gear rim is situated on the ring gear which has an additional internal toothing for a sliding clutch. This clutch contains two internal pawls (for the planet carrier) and two external pawls (for the ring gear AND the shell). For the hill gear the sliding clutch is pushed out of the toothing of the ring gear and thus the hub shell is driven by the planet carriers engaging with the internal pawls of the clutch. When the clutch gets pulled into the toothing of the ring gear the hub shell is driven directly by the external pawls of the clutch. The pawls of the planet carrier rotate slower and will be ‘overrun’.
These boys from Sachs were pretty clever, weren’t they? But we’ve seen through.

Orbit planetary gear train
Orbit gear unit
Orbit clutch

One simple planetary gear train – bidirectional

Countless 3-speed hubs all over the world are based upon the principle of a simple planetary gear train being operated in both directions. Its functionality was already presented in our other sections. The first 3-speed hub of the world was already produced in 1902 – from ‘The Three Speed Gear Syndicate Company Ltd’. All 3-speed hubs with simple gear train share a driver, i.e. the sprocket is not situated upon a gear component but on a driver transmitting the power flow to the gear train via an external clutch. In all 3-speed hubs the ring gear is the input for achieving the hill gear causing the planet carrier to rotate slower as output member. The hub manufacturers applied different principles for deactivating the pawls of the ring gear in that case: Sturmey-Archer pretty often pressed the pawls radially inwards – deactivating them – by means of the radiating clutch. Fichtel & Sachs, however, often pulled the ring out of the internal toothing of the hub shell.

Sturmey-Archer model K
Sturmey-Archer K pawls activated
Sturmey-Archer K pawls deactivated


In 1912 a 4-speed hub was produced by Fichtel & Sachs containing a stepped planetary gear train and a simple gear train. Unfortunately it was not very successful. Nowadays almost every hub providing more than three speeds comprises at least one stepped planetary gear train. The following sections will explain the varied range of opportunities of these gear unit types.

One non-shiftable stepped gear train – unidirectional

A stepped planetary gear trains allows achieving bigger or smaller gear ratios in a small space as we already know. In the Sturmey-Archer fixed gear hub Model TC thus a very close ratio and a direct gear is achieved. For the hill gear the ring gear – having no built-in pawls – is driven by the radiating clutch of the driver and the hub shell is driven by the slower rotating planet carrier. For achieving a direct gear the radiating clutch engages with the cams of the planet carrier. The ring gear also rotates, but disengaged due to the missing pawls.

Sturmey-Archer model TC
Sturmey-Archer TC stepped planetary train
Sturmey-Archer TC clutch

One non-shiftable stepped gear train – bidirectional

This gear version works like the standard 3-speed geared hub besides having a stepped gear train. In the Sturmey-Archer model AM in this way a closer gear ratio is achieved in contrast to the well-known model AW. The ring gear meshes with the small planet wheel- like in the TC hub as well. In some hub series Sturmey-Archer offered a version with wide ratio (simple gear train) and a version with close ration (stepped planetary gear train).

Sturmey-Archer model AM
Sturmey-Archer AM driver at planetary carrier
Sturmey-Archer stepped gear train

One shiftable stepped planetary gear train – unidirectional

Prior to the second world war Fichtel & Sachs offered a 3-speed hub with speed reducing ratio only – following the famous Doppeltorpedo. This interesting hub – meanwhile becoming quite rare – had two sun gears which could be slid alternately into two toothings of the axle. The ring gear was driven thus always leading to speed reducing gear ratios at the planet carrier. Additionally the small sun could be inserted into an internal toothing of the planet carrier for achieving a direct gear. The Shimano Nexus Inter 4 with rotary gear selector is another hub – with speed increasing ratio, however. Driving the large triple stepped planet carrier achieves three speed increasing gear ratios at the ring gear as output. The direct gear is obtained when all of the three suns are switched freely rotatable – the gear train is deactivated and the ring gear cannot transfer any torque. The power flow is now transferred 1:1 from the planet carrier to the hub shell.

Sachs model 29
Sachs model 29 suns and clutches
Shimano Nexus Inter 4

One shiftable stepped planetary gear train – bidirectional

Lots of 5 and 7-speed hubs contain this transmission type, driving the ring gear or the planet carrier by using a driver. Sturmey-Archer already launched a 5-speed hub in 1966 using this design. The advantage of a bidirectional stepped gear train is its simple structure. A disadvantage, however, is the dependency of the ratios of hill gears to the fast gears, since the same internal gear is driven forward and backward. Shimano designed a 7-speed hub with two stepped planetary gear trains connected in series in order to achieve a better grading of the single speeds.

Sturmey-Archer S5
Sturmey-Archer S5 small sun fixed
Sturmey-Archer S5 suns and clutches


Combining several gear trains leads to almost innumerable possibilities – mostly they are connected in series. With its ‘Tricoaster hub Sturmey-Archer in 1905 launched a hub containing two independent simple gear trains and other similar hubs would follow. The highest of all feelings seems to be the speedhub from Rohloff with its three stepped planetary gear trains connected in series.

Multiple single gear trains connected in series

When several gear trains are connected in series the total ratio will be the multiplication of the single ratios. Model 53 from Fichtel & Sachs contained two identical gear trains being connected by a common ring gear. Gear train 1 was driven by planet carrier 1 and geared up with i=1,33 at the common ring gear as output. Gear train 2 was driven by the common ring gear and geared down with i=0,75 at planet carrier 2 as output to the hub shell. Both planet carriers had internal toothings allowing the respective sun gear to be inserted for achieving a direct gear. Speed 1 was generated when coupling sun 1 with planet carrier 1 (i=1) and sun 2 being fixed (i2=0,75) leading to a total ratio of i=1×0, 75=0,75 (hill gear). For achieving speed 2 sun 2 was also coupled with planet carrier 2 thus leading to a total ratio of i=1×1=1 (direct gear). Speed 3 was generated when fixing sun 1 leading to a total ratio of i=1,33×1=1,33 (Fast gear). This principle was also applied in some historic Sturmey-Archer hubs before 1920. The contemporary 8-speed hub X-RF (from Sturmey-Archer) contained three simple gear trains connected in series. Every single gear train geared up with a specific gear ratio and it contained a direct gear additionally. Skillful combining of the three single ratios lead to an equal
graduation of the total ratio. Due to technical problems this hub was replaced by its successor X-RF8(W), however, which will be described in the following paragraph.

Sachs model 53 two planetary gear trains
Sachs model 53 planet carrier 1 and 2
Sachs model 53 sun gears

Simple and stepped gear trains connected in series

The model X-RF8(W) from Sturmey-Archer contains two simple planetary trains (set 1 and 3) and a double stepped planetary gear train (set 2), all of them are connected in series. Every planetary gear trains is driven at its planet carrier and has its output at the ring gear, which is connected with the ring gear of the following set. The hub shell is driven by the ring gear of the last set, in that way that innovative hub only gears up. Another feature presents the planet carrier with two pawls meshing with the associated ring gear’s teeth. When fixing a sun gear of a planetary gear train the ring gear rotates faster – as you know – and ‘overruns’ the pawls. If the sun rotates freely (deactivated gear) the ring gear tries to stand still but it is driven by the pawls with a ratio of 1:1 (direct gear). The clutches of the suns are interesting as well: Every single sun gear has an internal toothing which can mesh with a corresponding extendible pawl which is situated within the axle. This clutch design is easier than the shift drum of the Shimano Nexus Inter 8. Once again Sturmey-Archer shows its inventiveness.

Sturmey-Archer planet carrier with pawls
Sturmey-Archer suns with internal toothing
Sturmey-Archer extendible pawls

Multiple stepped gear trains connected in series

But don’t be afraid – Shimano was innovativ as well: the Nexus Inter 7 contains two stepped planetary gear trains connected in series. This hub comprises a rotary gear selector instead of toggle chains or indexing pins and was launched in 1992. The first gear train gears down (i1=0,63 and i2=0,74), it’s ring gear 1 is driven by a driver and planet carrier 1 as output is connected with planet carrier 2. The second gear train with planet carrier 2 as input gear up (i3=1,36 and i4=1,55). Planet carrier 2 and ring gear 2 is the output of the hub. It is interesting how the 7 speeds are realized by fixing or releasing the 4 suns and by means of a clutch within the driver:

Speed 1: Stage 1 of train 1 with i1=0,63 is activated, the suns of train 2 remain rotatable (deactivated gear). Thus the hub shell is driven by planet carrier 2 with the most speed reducing ratio i=0,63.

Speed 2: Stage 2 of train 1 with i2=0,74 is activated, train 2 remains deactivated. Thus the hub shell is driven by planet carrier 2 with the ratio i=0,74.

Speed 3: Stage 1 of train 1 gets activated again with i1=0,63, but now stage 1 of train 2 is shifted (i2=1,36). Thus the hub shell is driven by ring gear 2 with i=0,63×1,36=0,84.

Speed 4: Stage 2 of train 1 gets activated with i1=0,74 and stage 1 of train 2 remains shifted (i2=1,36). Thus the hub shell is driven with i=0,74×1,36=0,99.

Speed 5: Stage 2 of train 1 remains activated with i1=0,74 and Stage 2 of train 2 with i4=1,55 is now shifted leading to a ratio of i=0,74×1,55=1,15 at the hub shell.

Speed 6: The driver now gets engaged with planet carrier 1, i.e. gear train 1 is a direct gear with i=1. In gear train 2 now stage 1 is activated with i4=1,36. Thus the hub shell rotates with i=1×1,36=1,36.

Speed 7: Gear train 1 remains in the direct gear and stage 2 of train 2 is activated with i4=1,55 thus leading to a total ratio of i=1×1,55=1,55.

Isn’t it interesting to find out the Nexus Inter 7 has no direct gear as all of the other hubs? The Nexus Inter 8 works similar but it has the following differences: planetary gear train 2 is triple stepped providing 3 gearing up stages, gear train 1 is non-shiftable thus providing only one speed reducing ratio. Furthermore, a direct gear is applied here.

Shimano Nexus 7 stepped gear train 1
Shinano Nexus 7 stepped gear train 2
Shimano Nexus 7 sun gear and clutch


Since 1936 several 3-speed and 4-speed hubs from Sturmey-Archer were produced providing remarkably close gear ratios. This was achieved by two differential coupled gear trains. Many of those hubs were very successful and technically pretty sophisticated.

Two permanently differential coupled gear trains

Besides being connected in series, planetary gear trains can also be coupled differential, as already explained in Differential planetary gear trains in the menu Planetary gears. Both historic models AR and AC from Sturmey-Archer comprise two simple gear trains being permanently differential coupled: gear train 1 is driven by the driver – as in common 3-speed hubs causing ring gear 1 and planet carrier 1 to drive the hub shell as output members. Sun gear 1, however, is not fixed but permanently coupled with planet carrier 2 of the second gear train thus rotating permanently. Gear train 2 is driven by ring gear 2 which is integrated within the hub shell. Planet carrier 2 (rotating slower) revolves around a fixed sun gear 2 and drives sun gear 1 (differential sun). The relationships are as follows:

Speed 1: Ring gear 1 is driven by the driver and planet carrier 1 drives the hub shell (with deactivated pawls of ring gear 1). The planet carrier 1 will rotate a bit faster – compared to a stationary sun – since the differential sun rotates as well (addition of velocities). The speed-down gearing will be closer/smaller compared to a standard 3-speed hub.

Speed 2: The hub shell is driven directly by ring gear 1 because its pawls are activated again now. This provides a direct gear.

Speed 3: The driver engages with planet carrier 1 and ring gear 1 drives the hub shell (fast gear). Ring gear 1 will rotate a bit slower – compared to a stationary sun – since the differential sun rotates as well (subtraction of velocities). The speed-up gearing will be closer/smaller compared to a standard 3 speed hub.

The AR hub is designed similar but ring gear 2 is not integrated in the hub shell but it is connected with planet carrier 1. Both hubs achieve considerably closer ratios (AC: 0,92/1/1,07) compared to common 3 speed hubs (about 0,73/1/1,36) by a differential coupling.

Sturmey-Archer AC gear unit
Sturmey-Archer AC planetary gear train 2
Sturmey-Archer AC toothing within shell

Two non-permanently differential coupled gear trains

Fichtel & Sachs had already placed a four speed hub on the market from 1912 to 1916 containing several planetary gear trains which will be one of the most expensive collector’s item in the world. Apart from that Fichtel & Sachs merely produced 2-speed and 3-speed hubs until the 1980’s. Sturmey-Archer, however, offered several 4-speed hubs from 1935 to 1970. Three models (AF, FM and FC) contained two differential coupled gear trains which were designed similar to the model AC, already described. Speed 2 to 4 was applied with the same principle as in model AC, e.g. by also driving sun gear 1 a closer speed reducing or speed increasing ratio was obtained compared to a common 3 speed hub. For speed 1, however, sun gear 1 was released from planet carrier 2 and was fixed at the axle. This led to a bigger speed reducing ratio again compared to the other speeds. These 4 speed hubs contain a second clutch in addition to the clutch within the driver in order to uncouple the differential sun gear from the second gear train and to push it into a toothing on the axle.

Model ASC – probably the most sought-after fixed gear hub among hub collectors – works on a similar principle. It is operated unidirectional, however, with speed reducing ratio only. For speed 1 sun gear 1 was fixed, for speed 2 the differential coupling was activated and speed 3 was the direct gear.

Sturmey-Archer FC clutch deactivated
Sturmey-Archer FC clutch activated
Sturmey-Archer FM planetary gear train 2

A differential gear train and two stepped gear trains

The differential planetary gear train 1 described previously includes the following features:

Transmission input 1: The differential sun gear which was driven with constant ratio by gear train 2.

Transmission input 2: The differential planet carrier or the differential ring gear optionally, because this gear train was operated bidirectional.

Transmission output: This has been the differential planet carrier or the differential ring gear respectively.

In the Fichtel & Sachs Elan there is a differential planetary gear train (gear train 3) which is driven by two stepped planetary gear trains, however. Both stepped gear trains have a shared planet carrier 1/2 which is driven by the sprocket.

Transmission input 1 is the differential sun gear 3 which is directly combined with the ring gear 2 of the double stepped planetary gear train 2. For achieving a direct gear, ring gear 2 can be joined to the shared planet carrier 1/2 by a clutch. To sum up, there are 3 ratios at transmission input 1: A direct gear and two speed increasing ratios.

Transmission input 2 is the differential planet carrier 3 which is directly combined with the ring gear 1 of the triple stepped planetary gear train 1. For achieving an additional direct gear, ring gear 1 meshes with pawls of the driven shared planet carrier 1/2. These pawls drive the ring gear 1 directly (1:1) when all sun gears of gear train 1 are released (deactivated gear). This design is applied similar to the gear trains in the Sturmey-Archer 8 speed hub. To sum up, there are 4 ratios at transmission input 2: a direct gear and three speed increasing ratios.

Transmission output is the differential ring gear 3 which drives the hub shell directly via pawls forming a superposition of the velocities of input 1 and input 2.

The speed ratios of the ring gear as output were already described in the menu Planetary gear trains in the paragraph Gear trains as output member. As you can see in the video it is possible to provide speed reducing AND speed increasing ratios at the ring gear. The large differential gear train 3 has a stationary gear ratio of i12=1,3 (ring gear with 114 teeth and sun gear with 88 teeth: i12=114/88=1,295). The differential planet carrier 3 must at least rotate 1/i12=0,77 times as fast as the differential sun gear in order to prevent the differential ring gear from standing still or rotating backwards. Clever combinations of the single ratios made it possible to reach 4 x3=12 speeds. Die first 2 gears had a speed reducing ratio, the third gear was a direct gear and all further speeds were fast gears. Sachs was pretty innovative using slewable cam rods for fixing or releasing all sun gears of the planetary gear trains 1 and 2. Unfortunately, this ‘hub monster’ was not very successful – but technically very brilliant.

Sachs Elan three gear trains
Sachs Elan ring gear 1
Sachs Elan Ring gear 2 / sun 3